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S12700 V200R010C00 Configuration Guide - Device Management

This document describes the principles and configurations of the Device Management features, and provides configuration examples of these features.
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CSS Split and MAD

CSS Split and MAD

CSS Split

After a CSS is set up, the master and standby MPUs of the CSS periodically send heartbeat packets to each other to maintain the CSS status. If a cluster cable, a CSS card, or an MPU fails or one switch is powered off or restarted, communication between the two switches is interrupted. When the heartbeat timeout timer (8s) expires, the CSS splits into two single-chassis CSS systems, as shown in Figure 8-14.

Figure 8-14  CSS split

Multi-Active Detection

Two member switches in a CSS use the same IP address and MAC address (CSS system MAC address). Therefore, after the CSS splits, it becomes two single-chassis CSSs using the same IP address and MAC address, because the two switches run the same configuration file (configuration file of the previous CSS). To prevent this situation, a mechanism is required to check for IP address and MAC address collision after a CSS split.

Multi-active detection (MAD) is a CSS split detection protocol. When a CSS splits due to a link failure, MAD provides split detection, multi-active handling, and fault recovery mechanisms to minimize the impact of a CSS split on services.

MAD Detection Modes

MAD can be implemented in direct or relay mode. The direct and relay modes cannot be configured together in the same CSS.
  • Direct mode

    In direct mode, CSS member switches use direct links over common network cables as dedicated MAD links. When the CSS is running normally, member switches do not send MAD packets. After the CSS splits, the member switches periodically send MAD packets over the MAD links to check whether multiple master switches exist.

    In direct mode, CSS member switches can be directly connected to an intermediate device or directly connected to each other:
    • Directly connected to an intermediate device (Figure 8-15): Each member switch has at least one MAD link connected to the intermediate device. This deployment can be used when member switches are far from each other.

    • Directly connected to each other (Figure 8-16): No intermediate device is deployed in this topology so that MAD will not be affected by failures of the intermediate device.
    • After configuring MAD in direct mode on an interface, do not configure other services on the interface.
    • A maximum of four direct MAD links can be configured between member switches to ensure reliability.
    • MAD packets are bridge protocol data units (BPDUs), so the intermediate device must be able to forward BPDUs. For details on how to configure this function, see Configuring Interface-based Layer 2 Protocol Transparent Transmission.
    Figure 8-15  MAD through direct links to an intermediate device
    Figure 8-16  MAD through direct links between member switches
  • Relay mode

    In relay mode, MAD relay detection is configured on an Eth-Trunk interface in the CSS, and the MAD detection function is enabled on an agent. Every member switch must have a link to the agent and these links must be added to the same Eth-Trunk. In contrast to the direct mode, the relay mode does not require additional interfaces because the Eth-Trunk interface can run other services while performing MAD relay detection.

    In relay mode, when the CSS is running normally, member switches send MAD packets at an interval of 30s over the MAD links and do not process received MAD packets. After the CSS splits, member switches periodically send MAD packets over the MAD links to check whether multiple master switches exist.

    You can use an independent relay agent (Figure 8-17) or use two CSS systems as each other's relay agents (Figure 8-18).

    • The relay agent is a switch that supports the MAD relay function. Currently, all the S12700 series switches support this function.

    • To implement MAD relay detection by using two CSS systems as each other's relay agent, configure different domain IDs for the two CSS systems. Member switches of a CSS form a CSS domain. A network may have multiple CSS domains, with different domain IDs.

    Figure 8-17  Single switch as the MAD relay agent
    Figure 8-18  Two CSS systems as MAD relay agents of each other

Multi-Active Handling

After a CSS splits, the MAD mechanism sets the new single-chassis CSS systems to Detect or Recovery state. The CSS in Detect state still works, whereas the CSS in Recovery state is disabled.

MAD handles a multi-active situation in the following way: When MAD detects two CSS systems (two master switches) in Detect state, if one switch does not have any SFU and the other switch has SFUs, MAD enables the switch without SFUs to enter the Recovery state. If both switches have SFUs, MAD allows only the switch with a higher CSS priority to work. (If the two switches have the same CSS priority, their MAC addresses and CSS IDs are compared in turn.) Then the other switch enters the Recovery state, and all its physical ports except the excluded ones are shut down to prevent the switch from forwarding service packets.

Fault Recovery

After the faulty link recovers, the CSS systems merge into one in either of the following ways:
  • The CSS in Recovery state restarts and merges with the CSS in Detect state. The service ports that have been shut down are restored to Up state, and the entire CSS recovers.
  • If the CSS in Detect state is also faulty before the faulty link recovers, remove this CSS from the network, start the CSS in Recovery state to switch service traffic to this CSS, and rectify the CSS system fault. After the CSS recovers, connect it to the network so that it can merge with the other CSS.
Updated: 2019-08-21

Document ID: EDOC1000142080

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